Analysis methods of leakage current luminescence in CMOS circuits

ABSTRACT

Disclosed are a method and system for analyzing leakage current luminescence in CMOS circuits. The method comprises the steps of collecting light emission data from each of a plurality of CMOS circuits, and separating the CMOS circuits into first and second groups. For the first group of CMOS circuits, the emission data from the CMOS circuits are analyzed, based on the presence or absence of leakage light from the CMOS circuits, to identify logic states for the CMOS circuits. For the second group of CMOS circuits, the emission data from the CMOS circuits are analyzed, based on modulation of the intensity of the light from the CMOS circuits, to determine values for given parameters of the circuits. These parameters may be, for example, temperature, cross-talk or power distribution noise.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to semiconductor integratedcircuit (IC) diagnostics, and, in particular, to methods of non-invasiveoptical IC diagnostics using light emission from leakage currents infield-effect transistors and MOS capacitors.

2. Background Art

The progress of microelectronics complicates IC diagnostics, making manyexisting methods obsolete. For example, electrical IC characterizationusing mechanical probes is difficult and will become more difficult dueto shrinking transistor sizes, multiple metal layers, and increasing ICcomplexity.

Mechanical probing faces the following major problems. First, the accessto deep submicron wires requires complex and expensive techniques likeFocused Ion Beam (FIB) for probe point creation. More important, theincreasing number of metal layers often makes impossible the access towires deep in the metal stack. Second, the load capacitance and/orresistance of mechanical probes limits the bandwidth of timingmeasurements.

Third, the proliferation of flip-chip IC packaging makes the access tothe front side of an IC impossible, and, consequently, renders this typeof probing obsolete.

Another example of electrical IC characterization is e-beam probing.This method provides better spatial and timing resolution while beingsignificantly more expensive. It suffers basically from the sameproblems as the previous technique: the increasing number of metallayers and the front side inaccessibility.

Photon emission microscopy (PEM) is an alternative method of ICdiagnostics. This method uses visible and near infrared photon emissionfrom transistors, pn junctions and similar structures to makeconclusions about the operation of an IC. PEM can use time integratingdetectors (such as CCD cameras, Focal Plane Arrays, etc.) to obtain thedata, as well as time-resolved detectors (such as multichannel platephotomultipliers, single photon avalanche diodes, photomultipliers,etc.). The PEM can be performed both from the front-side and back-sideof an IC.

The back-side PEM enjoys increasing interest due to the fact that itcircumvents the two major problems of other techniques: multiple metallayers and flip-chip packaging.

Until recently, the only source of light emission from MOSFETs was hotelectron radiation from saturated devices. All existing PEM techniquesfor MOSFETs are based on this type of light emission. As the size ofMOSFETs decreases, two types of parasitic leakage currents becomeincreasingly important—gate tunneling current (this type of current isalso present in MOS capacitors) and OFF-state drain to source current.Each type of leakage current results in photon emission or, simply,leakage light. In general, the leakage light increases with the increaseof leakage current as well as the increase of the voltage differenceapplied to the device. The leakage light also depends on the devicetemperature, it increases with the increase of temperature.

The analysis of leakage light may provide valuable insights into opticalIC diagnostics. There is, therefore, a need in the art of PEM analysisof ICs for methods to extract information about IC operation fromleakage current emission measurements.

SUMMARY OF THE INVENTION

An object of this invention is to provide a method and system ofanalyzing leakage current luminescence in CMOS circuits.

Another object of the invention is to extract information about ICoperations from leakage current emission measurements.

In accordance with a first aspect of the invention, with a method andsystem are provided for analyzing leakage current luminescence in CMOScircuits. The method comprises the steps of collecting light emissiondata from each of a plurality of CMOS circuits, and separating the CMOScircuits into first and second groups. For the first group of CMOScircuits, the emission data from the CMOS circuits are analyzed, basedon the presence or absence of leakage light from the CMOS circuits, toidentify logic states for the CMOS circuits. For the second group ofCMOS circuits, the emission data from the CMOS circuits are analyzed,based on modulation of the intensity of the light from the CMOScircuits, to determine values for given parameters of the circuits. Forexample, the emission data from a CMOS circuit in the first group may beanalyzed to determine the logic states of individual switching deviceson the circuit or of the whole circuit. Also, the emission data from thesecond group of circuits may be analyzed to determine the temperature,cross-talk or power distribution noise on those circuits.

In accordance with a second aspect of the invention, methods areprovided for using photon emission microscopy to measure thetemperature, cross-talk and power distribution noise on CMOS circuits.More specifically, in accordance with this aspect of the invention, amethod is provided for measuring the temperature of a CMOS circuit. Thismethod comprises the steps of determining a relationship between theintensity of leakage light from the CMOS circuit and the temperature ofthe circuit, and using photon emission microscopy to measure photonsemitted from the CMOS circuit during a defined time period. This methodcomprises the further steps of analyzing the photon measurements made bythe photon emission microscopy to determine the intensity of lightemitted from the CMOS circuit during the defined time period, andcomparing said determined intensity with said determined relationship todetermine the temperature of the CMOS circuit.

A method is provided of measuring cross-talk noise in a CMOS circuit.This method comprises the steps of determining a relationship betweenleakage light from the circuit with a given voltage in the circuit,using photon emission microscopy to measure photons emitted from a givenarea of the CMOS circuit during a defined time period, analyzing thephoton emission measurements made using the photon emission microscopy,and comparing the analyzed photon emission measurements with thedetermined relationship to provide quantitative information about crosstalk noise in said given area.

A method is provided for characterizing a power supply noise on a CMOScircuit. This method comprises the steps of providing a set ofcalibration curves that relate leakage light from the circuit to a givenvoltage in the circuit, using photon emission microscopy to measurephotons emitted from a given area of the circuit during a defined timeperiod, and comparing photon measurements made by the photon emissionmicroscopy with the calibration curves to identify transient variationsin said power supply.

Further benefits and advantages of the invention will become apparentfrom a consideration of the following detailed description, given withreference to the accompanying drawings, which specify and show preferredembodiments of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a procedure for classifying the applications of leakagecurrent light analysis.

FIG. 2 depicts a semiconductor device and leakage current therefrom.

FIG. 3 is a time diagram showing the voltage applied to, and theluminescence levels of leakage light from, the semiconductor device ofFIG. 2.

FIG. 4 illustrates logic state detection using time-integratingdetectors.

FIG. 5 shows a procedure to determine device temperature using themeasurement of leakage light.

FIG. 6 illustrates a procedure to measure cross-talk noise.

FIG. 7 depicts a method for using leakage light to characterize powerdistribution noise.

FIG. 8 shows schematically an apparatus that may be used in the practiceof this invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 is an explanatory diagram classifying the applications of leakagecurrent light analysis 101. There are two major types of applicationspossible—those, represented at 102, that depend on the presence orabsence of leakage current light, and those, represented at 103, thatdepend on modulation of intensity of leakage light by variousparameters. These parameters include the dependence on the devicetemperature 104 and electrical parameters 105 such as voltage(drain-to-source, gate-to-source, substrate voltage, etc) and current(drain-to-source, gate-to-channel, etc.).

The presence or absence of leakage light from a particular MOSFET allowsthe detection of its logical state (ON, OFF).

In turn, the knowledge of the logical states of MOSFETs allows thereconstruction of the logical state of an entire IC.

Thus, the logical state of an IC can be determined by measurements ofleakage light from MOSFETs.

FIG. 2 illustrates this method for CMOS Inverter 201 using OFF-stateleakage light. The leakage light 202 from nFET 203 is emitted only whenthe Inverter is in logic state 1 (i.e. input 206 Vin=0, output 207Vout=1). The leakage light from pFET is emitted only when the Inverteris in logic state 0 (input 206 Vin=1, output 207 Vout=0). The leakagelight emission from both types of devices is persistent (i.e. thedevices emit the light as long as the Inverter 201 is in a definitestate), unlike the light emitted by MOSFETs in saturation, which istraditionally used by PEM. This type of light emission occurs when theInverter 201 changes its logical state. On FIG. 3, the peak 209corresponds to the switching of the pFET, and the peak 210 correspondsto the switching of the nFET.

Similar considerations show that the state of Inverter 201 can bedetermined using gate tunneling light. Moreover, both types of leakagelight can be used to determine the state of an arbitrary CMOS gate.

The Logic State Detection of an IC operating at speed can be performedby measuring leakage light from MOSFETs using a time-resolved detector.

Also, the Logic State Detection of a stopped IC can be performed byusing a time-integrating detector as shown on FIG. 4. The clock 301 isapplied at nominal speed to an IC until a clock cycle of interest isreached. Then the clock is stopped to keep the logical state of the ICand the image of leakage currents is taken. During this time, the supplyvoltage Vdd (303) can be raised 304 to increase the intensity of leakagelight. After the completion of image acquisition, the clock is appliedagain until the next clock cycle of interest is reached, another imageis taken, etc.

As shown on FIG. 1, the second type of leakage light applications isbased on the fact that a number of important parameters can modulate theintensity of that light. Once the dependence of the leakage light on aparticular parameter is known, that dependence can be inverted toextract the value of the parameter from the leakage light measurements.

A first important parameter is the device temperature. This fact can beused to determine individual device temperature using the measurementsof leakage light emitted by this device.

FIG. 5 illustrates this method. An IC operates at given clock speed 401.The temperatures of individual devices are increased due to theirswitching activity. The light emitted by an individual transistorconsists of peaks 403 that correspond to its switching activity and“baseline” leakage light 404. Then the clock is stopped at 402. Thedevice temperature decreases and eventually equals ambient temperature,which results in the change of leakage light, as represented at 405 (inthis particular example we assume it decreases too). The temperature ofthe operating device can be extracted using calibration curves thatrelate the leakage light emission with temperature. The calibrationcurves can be measured for single devices in a temperature controlledenvironment.

Alternatively, the time-integrating detector can be used to measure thetemperatures distribution among the devices provided that imageacquisition begins immediately after the clock is stopped 402 and endsbefore the leakage lights reach their ambient value 405. In the casewhen the exposition time is not enough, the procedure of stopping theclock can be repeated.

Various electrical parameters modulate the leakage light. One importantapplication of this fact is the possibility of optical signal integrityanalysis (108 on FIG. 1). The issue of signal integrity includes suchfactors as crosstalk noise, power distribution noise, etc.

A method to measure crosstalk noise is illustrated on FIG. 6. The“victim” wire 501 is in logical state 0. It is connected to the input ofinverter 503 and is coupled to nearby “aggressor” wires 502 through avariety of coupling mechanisms 505 (inductive, capacitive). The leakagelight from inverter's nFET is relatively small. The aggressor wiressimultaneously transition from logical 0 to 1. The potential of thevictim wire 501 increases, which leads to the increase of leakage light504. The use of calibration curves that relate the leakage light,measured using a time-resolved detector, with the gate-to-source voltagein an nFET, will provide quantitative information about the crosstalknoise in the victim wire.

Time-integrating detectors can also be effectively used for crosstalknoise analysis. Two input patterns are chosen. The first one correspondsto the “minimum” crosstalk (i.e. no transitions in victim and aggressorlines). The second one corresponds to the “maximum” crosstalk (i.e.simultaneous transitions in aggressor lines, no transitions in thevictim line). Photon emission images are taken for both patterns and thedifference image is computed. The intensity of light emission from theinverter 503 on the difference image is used to detect if there is anysignificant crosstalk.

Similarly, the dependence of leakage light in MOSFETs and MOS capacitorson power supply voltage can be used to characterize the powerdistribution noise. FIG. 7 illustrates the method using OFF-stateleakage light from nFET. When the inverter 601 is in logic state 1, thedrain-to-source voltage across its nFET is close to power supply voltageVdd 602. Thus, the leakage light from this nFET 603 is modulated by Vdd.The transient variations of Vdd cause corresponding variations of theleakage light, which, in turn, can be measured by any time-resolvedphoton detector. The actual voltage waveforms can be extracted from thismeasurement by using calibration curves. The latter relate leakage lightfrom an individual FET to the drain-to-source voltage, and, possibly,the temperature of the device (which can be measured by the abovedescribed method).

Similarly, the power noise analysis can use the leakage light emissionfrom pFETs, gate tunneling light from FETs of any types as well thatfrom MOS capacitors.

FIG. 8 shows schematically an apparatus that may be used to obtain timeresolved optical images and optical waveforms for use in this invention.The apparatus of FIG. 8 comprises a dark space 801, a microscope 802having at least its objective disposed in the dark space 801, an imagingoptical detector 803 such as an imaging microchannelplate photomultiplier tube, a readout 804 for the imaging detector 803 capable ofproviding a series of optical images of the emission, each comprising aseparate time interval. Alternatively, readout 804 is capable ofproviding a series of optical waveforms detected by detector 803, whereeach waveform is obtained from a different portion of the image. Asemiconductor integrated circuit to be evaluated 805 is mounted in thedark space at the focus of the microscope objective and powered by asuitable power source. Ideally, detector 803 is capable of countingsingle photons. For a photon-counting detector, the portion of readout807 that provides temporal information can be of the type described inthe literature as photon timing. Other optical sampling techniques suchas up-conversion or a streak camera may also be used.

In a specific implementation of the apparatus of FIG. 8, the imagingoptical detector 803 was an imaging microchannelplate photomultipliertube (MEPSICHRON) available from Quantar Technology Inc. of Santa Cruz,Calif. This type of photomultiplier has been shown to be capable ofabout 100 psec time resolution. This is far shorter than the timeconstants of the power supply and the thermal time constant of the chip.The required time constants for these measurements are those of thepower supplies and the chips. The readout 804 was a three dimensionalmultichannel analyzer, which stored information about both the position(x,y) and time (t) for each photon detected. The photons detected over atime interval may be displayed as an image. Each such image shows whichdevices of circuit 805 are in the process of changing logic statesduring that time interval.

While it is apparent that the invention herein disclosed is wellcalculated to fulfill the objects stated above, it will be appreciatedthat numerous modifications and embodiments may be devised by thoseskilled in the art, and it is intended that the appended claims coverall such modifications and embodiments as fall within the true spiritand scope of the present invention.

1. A method of analyzing leakage current luminescence in CMOS circuits, comprising the steps of: collecting light emission data from each of a plurality of CMOS circuits; separating the CMOS circuits into first and second groups; for the first group of CMOS circuits, analyzing the emission data from the CMOS circuits, based on the presence or absence of leakage light from the CMOS circuits, to identify logic states for the CMOS circuits; and for the second group of CMOS circuits, analyzing the emission data from the CMOS circuits, based on modulation of the intensity of the light from the CMOS circuits, to determine values for given parameters of the circuits.
 2. A method according to claim 1, wherein: each of the CMOS circuits includes a multitude of individual switching devices; and the step of analyzing the emission data from the first group of CMOS circuits includes the step of analyzing emission data from one of the CMOS circuits of said first group to determine the logic states of each of at least some of the switching devices of said one of the CMOS circuits.
 3. A method according to claim 2, wherein the step of analyzing the emission data from the first group of CMOS circuits includes the further step of reconstructing the logical state of said one of the CMOS circuits from the determined logic states of said at least some of the switching device.
 4. A method according to claim 1, wherein the step of analyzing the emission data from the first group of CMOS circuits includes the step of analyzing the emission data from one of the CMOS circuits of said first group when said one of the CMOS circuits is operating at a defined speed.
 5. A method according to claim 1, wherein the step of analyzing the emission data from the first group of CMOS circuits includes the step of analyzing the emission data from one of the CMOS circuits of said first group when said one of the CMOS circuits is stopped.
 6. A method according to claim 1, wherein the step of analyzing the emission data from the second group of CMOS circuits includes the steps of, for at least one of the CMOS circuits of said second group, determining a relationship between the given parameter and the intensity of light from the one of the CMOS circuits; measuring the intensity of light emitted from the one of the CMOS circuits; and comparing said measured intensity with said determined relationship to determine the value for the given parameter on said one of the CMOS circuits.
 7. A method according to claim 1, wherein the given parameter is selected from the group comprising temperature, crosstalk noise, and power supply noise.
 8. A method according to claim 1, wherein: each of the CMOS circuits includes a multitude of individual switching devices; and the step of analyzing the emission data from the second group of CMOS circuits includes the step of analyzing the emission data from one of the CMOS circuits of the second group to determine a temperature of one of the individual switching devices of said one of the CMOS circuits.
 9. A method according to claim 1, wherein: each of the CMOS circuits includes a multitude of individual switching devices; and the step of analyzing the emission data from the second group of CMOS circuits includes the step of analyzing emission data from one of the CMOS circuits of the second group to determine a temperature distribution among the switching devices of said one of the CMOS circuits.
 10. A system for analyzing leakage current luminescence CMOS circuits, comprising: means for collecting light emission data from each of a plurality of CMOS circuits; means for analyzing the emission data from a first group of CMOS circuits, based on the presence or absence of leakage light from the CMOS circuits, to identify logic states for the CMOS circuits of said first group; and means for analyzing the emission data from a second group of CMOS circuits based on modulation of the intensity of the light from the CMOS circuits, to determine values for given parameters of the CMOS circuits of said second group.
 11. A system according to claim 10, wherein: each of the CMOS circuits includes a multitude of individual switching devices; and the means for analyzing the emission data from the first group of CMOS circuits includes means for analyzing emission data from one of the CMOS circuits of said first group to determine the logic states of each of at least some of the switching devices of said one of the CMOS circuits
 12. A system according to claim 11, wherein the means for analyzing the emission data from the first group of CMOS circuits further includes means for reconstructing the logical state of said one of the CMOS circuits from the determined logic states of said at least some of the switching device.
 13. A system according to claim 10, wherein the means for analyzing the emission data from the first group of CMOS circuits includes means for analyzing the emission data from one of the CMOS circuits of said first group when said one of the CMOS circuits is operating at a defined speed.
 14. A system according to claim 10, wherein the means for analyzing the emission data from the first group of CMOS circuits includes means for analyzing the emission data from one of the CMOS circuits of said first group when said one of the CMOS circuits is stopped.
 15. A system according to claim 10, wherein the means for analyzing the emission data from the second group of CMOS circuits includes: means for determining a relationship between the given parameter and the intensity of light from at least one of the CMOS circuits of the second group; means for measuring the intensity of light emitted from the one of the CMOS circuits; and means for comparing said measured intensity with said determined relationship to determine the value for the given parameter on said one of the CMOS circuits.
 16. A system according to claim 10, wherein the given parameter is selected from the group comprising temperature, crosstalk noise, and power supply noise.
 17. A system according to claim 10, wherein: each of the CMOS circuits includes a multitude of individual switching devices; and the means for analyzing the emission data from the second group of CMOS circuits includes means for analyzing the emission data from one of the CMOS circuits of the second group to determine a temperature of one of the individual switching devices of said one of the CMOS circuits.
 18. A system according to claim 10, wherein: each of the CMOS circuits includes a multitude of individual switching devices; and the means for analyzing the emission data from the second group of CMOS circuits includes means for analyzing emission data from one of the CMOS circuits of the second group to determine a temperature distribution among the switching devices of said one of the CMOS circuits.
 19. A program storage device readable by machine, tangibly embodying a program of instructions executable by the machine to perform method steps for analyzing leakage current luminescence in CMOS circuits, said circuits being separated into first and second groups based on light emission data collected from the CMOS circuits, said method steps comprising: analyzing the emission data from the first group of CMOS circuits based on the presence or absence of leakage light from the CMOS circuits to identify logic states for the CMOS circuits; and analyzing the emission data from the second group of CMOS circuits based on modulation of the intensity of the light from the CMOS circuits to determine values for given parameters of the circuits.
 20. A program storage device according to claim 19, wherein: each of the CMOS circuits includes a multitude of individual switching devices; and the step of analyzing the emission data from the first group of CMOS circuits includes the step of analyzing emission data from one of the CMOS circuits of said first group to determine the logic states of each of at least some of the switching devices of said one of the CMOS circuits.
 21. A program storage device according to claim 19, wherein the step of analyzing the emission data from the second group of CMOS circuits includes the steps of, for at least one of the CMOS circuits of said second group, determining a relationship between the given parameter and the intensity of light from the one of the CMOS circuits; measuring the intensity of light emitted from the one of the CMOS circuits; and comparing said measured intensity with said determined relationship to determine the value for the given parameter on said one of the CMOS circuits.
 22. A method of measuring the temperature of a CMOS circuit, comprising the steps of: determining a relationship between the intensity of leakage light from the CMOS circuit and the temperature of the circuit; using photon emission microscopy to measure photons emitted from the CMOS circuit during a defined time period; analyzing the photon measurements made by the photon emission microscopy to determine the intensity of light emitted from the CMOS circuit during the defined time period; and comparing said determined intensity with said determined relationship to determine the temperature of the CMOS circuit.
 23. A method according to claim 22, wherein the CMOS circuit includes a multitude of individual switching devices, and the comparing step includes the step of determining the temperature distribution among said devices.
 24. A method of measuring cross-talk noise in a CMOS circuit, comprising the steps of: determining a relationship between leakage light from the circuit with a given voltage in the circuit; using photon emission microscopy to measure photons emitted from a given area of the CMOS circuit during a defined time period; analyzing the photon emission measurements made using the photon emission microscopy; and comparing the analyzed photon emission measurements with the determined relationship to provide quantitative information about cross talk noise in said given area.
 25. A method according to claim 24, wherein the step of determining a relationship between leakage light from the circuit and a given voltage in the circuit includes the steps of: obtaining a first photon emission image for a pattern representing a low level of cross talk; obtaining a second photon emission image for a pattern representing a high level of cross talk; and computing the difference between the first and second photon emission images.
 26. A method of characterizing noise from a power supply on a CMOS circuit, comprising the steps of: providing a set of calibration curves that relate leakage light from the circuit to a given voltage in the circuit; using photon emission microscopy to measure photons emitted from a given area of the circuit during a defined time period; and comparing photon measurements made by the photon emission microscopy with the calibration curves to identify transient variations in said power supply.
 27. A method according to claim 26, wherein: the power supply applies a drain-to-source voltage to transistors of the circuit; and the step of providing a set of calibration curves includes the step of providing a set of calibration curves that relate leakage light from an individual transistor of the circuit to the drain-to-source voltage applied to said individual transistor. 